The invention relates to an electronic circuit intended to feed a coil sending out a magnetic field, having a first input terminal for receiving a power-supply voltage, a second input terminal for receiving a periodic control signal in a predefined frequency range, an output terminal for generating an output current in the said sending coil connected between the said output terminal and earth, in such a way as to convert the said periodic control signal into a periodic magnetic field sent out by the coil.
Such a circuit is intended particularly to feed a magnetic-field sending coil serving as an antenna in a system, called xe2x80x9chands-freexe2x80x9d system, for access to an enclosed space, this enclosure possibly being a motor vehicle, for example. Such a system may also allow or prevent a vehicle being started. Such a system generally includes a recognition device having a sending antenna in the form of a coil, which sends out a periodic magnetic field in order to carry on an exchange of data with an identification unit to be authenticated. Conventionally, the circuit for feeding the sending coil, which is powered by a battery of the vehicle, receives a control signal having a given frequency and converts this signal into a current which passes through the coil so that the latter sends out a magnetic field having the same frequency as the control signal.
In the prior art relating to xe2x80x9chands-freexe2x80x9d access systems, it is generally observed that it is difficult to obtain high-quality data transmission between the recognition device and the identification unit. This is because the radiation pattern of a magnetic field in air is difficult to control, having regard particularly to the disturbances which the metal body-work of the vehicle introduces, which have an influence on the sending-antenna electrical characteristics, and having regard to the siting of the sending antenna which is subject to ergonomics imperatives.
One way of resolving this problem consists in optimizing the siting and the design of the sending coil so that the radiation pattern is satisfactory.
The defect in this solution is that it gives rise to increased costs for manufacturing the recognition device.
The object of the invention is to remedy this drawback by proposing a circuit for feeding the sending coil which enhances the quality of the data transmission.
To that end, the subject of the invention is an electronic circuit intended to feed a coil sending out a magnetic field, having a first input terminal for receiving a power-supply voltage, a second input terminal for receiving a periodic control signal in a predefined frequency range, an output terminal for generating an output current in the said sending coil connected between the said output terminal and earth, so as to convert the said periodic control signal into a periodic magnetic field sent out by the coil, characterized in that a series RLC circuit is connected between the said output terminal and earth so as to form a tuned antenna with the sending coil.
With such a construction, the magnetic field sent out is sinusoidal, and its range is increased, such that the sensitivity of a receiver as an identification unit is enhanced.
According to one preferred embodiment of the invention, in which the feed circuit comprises a volt-age-booster circuit, in order to use a magnetic-field sending antenna having a low quality factor or featuring electrical characteristics which are disturbed by the metal bodywork of the vehicle, a recognition device is obtained for which the overall cost of manufacture is reduced. Furthermore, in this embodiment, the voltage delivered by the voltage-booster circuit does not vary with fluctuations in the power-supply voltage supplied by a battery of the vehicle, such that the magnetic field sent out has a constant range. In this embodiment, the voltage delivered by the voltage-booster circuit may be set up in the course of a calibration operation.
According to another embodiment of the invention, in which the feed circuit comprises a control circuit, the peak intensity of a current which passes through the sending coil is constant and does not vary with fluctuations in the power-supply voltage supplied by the battery of the vehicle, such that the magnetic field sent out by the sending coil has a constant range.
According to yet another particular embodiment in which the feed circuit comprises a control circuit, it is then possible deliberately to alter the range of the magnetic field sent out by the sending coil so that the recognition device can, for example, better evaluate the position of an identification unit to be authenticated.